Many vessels in animals transport fluids from one bodily location to another. In some vessels, natural valves are positioned along the length of the vessel to permit fluid flow in a substantially unidirectional manner along the length of the vessel.
For example, by use of a closed circulatory system, animal bodies use many internal organs and vessels to transport fluids from one bodily location to another. Components of the circulatory system include the heart, blood vessels, and blood. The heart has valves that regulate the flow of blood in the atria and the ventricles. Three examples of blood vessels are arteries, veins, and capillaries. Whereas arteries transport blood to organs throughout the body (i.e., away from the heart), veins carry blood back to the heart. Structurally, capillaries have an inner endothelium surrounded by a membrane, while arterial and venal walls have three layers: connective tissue forms the outer layer, while smooth muscle having elastic fibers forms the middle layer, and there is an innermost endothelium layer.
Mammalian veins have naturally occurring valves positioned along the length of the vessel. These valves act as one-way check valves that open to permit the flow of fluid in a first direction (e.g., muscles contract, squeeze the veins, and the valves—flaps of tissue—keep blood moving toward the heart (antegrade flow)), and quickly close upon a change in pressure, such as a transition from systole to diastole, or when muscles relax or stop contraction, to prevent fluid flow in a reverse direction, i.e., retrograde flow.
Natural valves may have a leakiness quality to them, allowing a relatively small quantity of fluid to flow in a reverse direction (i.e., a second direction opposed to the first direction; retrograde flow) when the valve is in closed position. It is believed that this leakiness limits the pooling of blood around the valve during periods of low pressure, which can reduce the formation of thrombus and, therefore, increase the effective lifetime of the valve.
While natural valves may function for an extended time, some may lose effectiveness, which can lead to physical manifestations and pathology. For example, venous valves are susceptible to becoming insufficient due to one or more of a variety of factors. Over time, the vessel wall may stretch, affecting the ability of the valve leaflets to close. Furthermore, the leaflets may become damaged, such as by formation of thrombus and scar tissue, which may also affect the ability of the valve leaflets to close. Once valves are damaged, venous valve insufficiency may be present and can lead to discomfort and possibly ulcers in the legs and ankles.
Current treatments for venous valve insufficiency include the use of compression stockings that are placed around the leg of a patient in an effort to force the vessel walls radially inward to restore valve function. Surgical techniques are also employed in which valves can be bypassed or replaced with sections of veins with competent valves.
Over recent years, a wide variety of minimally invasive techniques and instruments for placement of intraluminal medical devices have been developed. Such treatment devices include stents, stent grafts, occlusion devices, infusion catheters and the like. Minimally invasive intravascular devices have especially become popular with the introduction of coronary stents to the U.S. market in the early 1990s. Coronary and peripheral stents have been proven to provide a superior means of maintaining vessel patency, and have become widely accepted in the medical community. Furthermore, the use of stents has been extended to treat aneurysms and to provide occlusion devices, among other uses.
Artificial valves have been proposed to replace damaged natural valves. One variety of such artificial valves consists of a stent supporting one or more valve leaflets. The leaflets are configured to allow flow in an antegrade direction and to restrict flow in a retrograde direction. One drawback to this type of valve is that the supporting stent contacts the wall of the vessel in the region of placement of the valve. This can result in irritation of the vessel wall, resulting in intimal hyperplasia and thrombosis.
In another variety of artificial valve, the valve leaflet is attached to and supported by the vessel wall. Such artificial valves do not include a support stent and offer the advantage of reduced irritation of the vessel wall in the region of attachment of the valve. However, the absence of a supporting stent can result in difficulties during the delivery of the valve to the site of attachment to the vessel wall and during the process of attachment of the valve leaflet to the wall.